[AMDGPU] Use StringRef::consume_front (NFC) (llvm#141442)

Author: kazutakahirata

llvm/lib/Target/AMDGPU/AsmParser/AMDGPUAsmParser.cpp

@@ -9231,8 +9231,7 @@ bool AMDGPUAsmParser::parseDimId(unsigned &Encoding) {
   Token += Suffix;
 
   StringRef DimId = Token;
-  if (DimId.starts_with("SQ_RSRC_IMG_"))
-    DimId = DimId.drop_front(12);
+  DimId.consume_front("SQ_RSRC_IMG_");
 
   const AMDGPU::MIMGDimInfo *DimInfo = AMDGPU::getMIMGDimInfoByAsmSuffix(DimId);
   if (!DimInfo)